10.10.1 Constructor initializers

All instance constructors (except those for class object) implicitly include an invocation of another instance constructor immediately before the constructor-body. The constructor to implicitly invoke is determined by the constructor-initializer:

• An instance constructor initializer of the form base(argument-listopt) causes an instance constructor from the direct base class to be invoked. That constructor is selected using argument-list and the overload resolution rules of Section 7.4.2. The set of candidate instance constructors consists of all accessible instance constructors contained in the direct base class (including any default constructor, as defined in Section 10.10.4). If this set is empty, or if a single best instance constructor cannot be identified, a compile-time error occurs.
• An instance constructor initializer of the form this(argument-listopt) causes an instance constructor from the class itself to be invoked. The constructor is selected using argument-list and the overload resolution rules of Section 7.4.2. The set of candidate instance constructors consists of all accessible instance constructors declared in the class itself. If this set is empty, or if a single best instance constructor cannot be identified, a compile-time error occurs. If an instance constructor declaration includes a constructor initializer that invokes the constructor itself, a compile-time error occurs.

If an instance constructor has no constructor initializer, a constructor initializer of the form base() is implicitly provided. Thus, an instance constructor declaration of the form

C(...) {...}

is exactly equivalent to

C(...): base() {...}

The scope of the parameters given by the formal-parameter-list of an instance constructor declaration includes the constructor initializer of that declaration. Thus, a constructor initializer is permitted to access the parameters of the constructor. For example:

class A
{
   public A(int x, int y) {}
}
class B: A
{
   public B(int x, int y): base(x + y, x - y) {}
}

An instance constructor initializer cannot access the instance being created. Therefore it is a compile-time error to reference this in an argument expression of the constructor initializer, as is it a compile-time error for an argument expression to reference any instance member through a simple-name.